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Soil factors affecting glyphosate efficacy in Lolium SPP.

Ncedana, Chwayita (2011-12)

Thesis (MScAgric)--Stellenbosch University, 2011.

Thesis

ENGLISH ABSTRACT: Glyphosate remains an important herbicide in weed control. This is due to several positive
attributes it has including sytemicity, wide spectrum of weed control and environmental
friendliness. Its efficacy and lack of residual activity are therefore important to ensure
adequate weed control without imposing hazards to the environment. Despite these favourable
attributes for weed control glyphosate has its shortcomings.
Evolution of resistance to glyphosate has been a major concern from 1996. However,
there are other factors that reduce the maximum potential of glyphosate. Any factors that
reduce glyphosate efficacy may result in the target plant being subjected to non-lethal
concentrations of glyphosate. This in turn may predispose the plants to developing herbicide
resistance. Some factors that may influence efficacy of glyphosate, and therefore be possibly
selecting for resistance were investigated in this study.
Although glyphosate is a postemergence herbicide, its efficacy is not exempt from the
effect of soil and nutrients in which the weeds occur. The possibility of this occurring was
investigated in a greenhouse study on ryegrass (Lolium spp.) In this study ryegrass was grown
in three soils: pure sand (SS), soil from pasture paddock (PS) and soil from crop field (CS).
The soils varied in nutrient composition and, although all were classified as sand, they had
varying proportions of sand, loam and clay. This investigation consisted of four experiments.
The first experiment was investigating the effect of growing a susceptible commercial
ryegrass cultivar on PS, CS and SS soils on the efficacy of glyphosate (360 g a.i. L-1
formulation) applied at five glyphosate application rates (GAR). The GARs were 0 (0x), 67.5
(1/8x), 135 (1/4x), 270 (1/2x) and 540 (1x) g a.i. ha-1. The second experiment investigated the
effect of growing a susceptible commercial ryegrass cultivar and a glyphosate resistant
ryegrass biotype on PS and CS soils on the efficacy of glyphosate. The application rates were
0 (0x), 270 (1/2x), 540 (1x), 1080 (2x) and 2160 (4x) g a.i. ha-1. The third and the fourth
experiments were similar to the first experiment except: The latter investigated the role of
nutrient content of irrigation water (pure water or balanced nutrient solution) and; the former
investigated the effect of soil activity (by covering the soil surface with cotton at the time of
spraying) of glyphosate with regard to the role it plays on efficacy of glyphosate. Our findings
showed that: i) soil affects the efficacy of glyphosate with more control (19% survivors) found in the PS soil compared to 50% and 62% survivors in CS and SS soils respectively, this effect
may be dependent upon the species resistance as; ii) the effect in the resistant ryegrass biotype
was reversed with about 95% of survivors in the PS soil compared to about 78% in CS soil;
iii) efficacy of glyphosate is influenced by the soil nutrient status and the nutrient content of
the irrigation water. This was shown by decrease in the control of ryegrass (100% survivors)
grown in SS soil when fed with pure water compared to 45% when nutrient fed. In PS soil
there was no significant effect. This was probably due to inherently higher nutrient content of
the PS soil; and iv) glyphosate efficacy is influenced by the amount of glyphosate reaching the
soil (absorbed through the roots). This was shown in PS soil where 1/8x GAR resulted in
93.3% survivors in covered soil compared to 60% in uncovered soil. A similar trend was also
observed at 1/4x GAR. An opposite effect was shown in SS soil with 0% and 40% survival at
1/4x GAR in the covered and uncovered soil respectively.
Glyphosate has been hailed as an environmentally friendly herbicide as it rapidly
degrades in soil and it sorbs on metals embedded in soil matrix. However, reports in the
literature have showed reduction in crop yield due to soil glyphosate residues. In these studies,
glyphosate phytotoxicity was found to be dependent on certain soil characteristics and nutrient
content. Following this, a greenhouse study was conducted to assess the phytotoxic activity of
glyphosate on a susceptible commercial ryegrass cultivar grown in PS, CS and SS soils.
Glyphosate was applied at 0 (G1), 540 (G2) and 3240 (G3) g a.i. ha-1. Ryegrass seedlings of
comparable size were transplanted into the soil at intervals of two hours, three weeks and four
weeks after glyphosate application referred to as TAS1, TAS2 and TAS3 respectively.
Evidence of soil glyphosate activity was shown by the decrease in percentage survival with
the application of glyphosate. This was significant in the SS soil where about 60% and 48%
survival in G1 and G2 GAR respectively was observed compared to about 100% in the
untreated control when transplanted three weeks after glyphosate application. The decrease in
percentage survival was time mediated with significant effect of G2 GAR shown at TAS 1
whereas at G3 GAR the effect was significant at TAS1 and TAS 2. At TAS 3 there was no
effect at all GARs. Similar trends were observed with dry mass and shoot length.
Trace metals required for normal plant growth have been implicated in the reduction of
glyphosate efficacy. This follows glyphosate’s original development as a metal chelator.
Glyphosate-trace metal antagonism has recently sparked interest following co-application in
glyphosate resistant soybeans. Molybdenum (Mo), an anion, may play a role at the
physiological level on the antagonism of glyphosate. A greenhouse assay was carried out where seedlings grown from seeds (of susceptible commercial ryegrass cultivar (S biotype)
and glyphosate resistant biotype (R biotype)) were grown with nutrient solutions containing
0x, 1x and 2x molybdenum (Mo) concentrations where 1x is 0.05 mg L-1 Mo. Glyphosate was
applied at 0 (0x), 135 (1/4x), 270 (1/2x), 540 (1x) and 1040 (2x) g a.i. ha-1 rates. In the R
biotype applying 2x Mo resulted in 0% survival in the R biotype at 1x GAR compared to 50%
and 90% survival at the same GAR with 0x and 1x Mo. In terms of dry mass and shoot length
the results did not show any conclusive trends.